The Rankine cycle is a thermodynamic cycle that converts heat into work. The heat is supplied externally to a closed loop, which usually uses water as the working fluid. This cycle generates about 80% of all electric power used throughout the world, and is used by virtually all solar, thermal, biomass, coal and nuclear power plants. It is named after William John Macquorn Rankine, a Scottish engineer and physicist (Jul. 5, 1820-Dec. 24, 1872). William Thomson (Lord Kelvin) and Rudolf Clausius were the founding contributors to the science of thermodynamics. Rankine developed a complete theory of the steam engine and, indeed, of all heat engines. His manuals of engineering science and practice were used for many decades after their publication in the 1850s and 1860s. He published several hundred papers and notes on science and engineering topics, from 1840 onward, and his interests were extremely varied, including, in his youth, botany, music theory and number theory, and, in his mature years, most major branches of science, mathematics and engineering. A Rankine cycle describes a model of a steam-operated forward heat engine most commonly found in power generation plants. The combustion of coal, natural gas and oil, as well as nuclear fission, commonly provides the heat for power plants employing the Rankine cycle. Rankine cycle power systems typically transform thermal energy into electrical energy. A conventional Rankine cycle power system employs the following four basic steps: (1) thermal energy is used, in a boiler, to turn water into steam; (2) the steam is sent through a turbine, which, in turn, drives an electric generator; (3) the steam is condensed back into water by discharging the remaining thermal energy in the steam to the environment; and (4) the condensate is pumped back to the boiler. In the ideal Rankine cycle, the expansion is isentropic and the evaporation and condensation processes are isobaric. However, the presence of irreversibilities in the real world lowers cycle efficiency. Those irreversibilities are primarily attributable to two factors.
The first is that during expansion of the gas, only a part of the energy recoverable from the pressure difference is transformed into useful work. The other part is converted into heat and is lost. The efficiency of the expander is stated as a percentage of work that would be performed by a theoretical isentropic expansion, in which entropy remains constant. The second cause is heat exchanger inefficiency caused by pressure drops associated with the long and sinuous paths that ensure good heat exchange, but lower the power recoverable from the cycle.
The efficiency of a Rankine cycle is a function of the physical properties of the working fluid. Without the pressure reaching super critical levels for the working fluid, the temperature range the cycle can operate over is quite small: turbine entry temperatures are typically 565° C. (the creep limit of stainless steel) and condenser temperatures are around 30° C. This gives a theoretical Carnot efficiency of about 63% compared with an actual efficiency of 42% for a modern coal-fired power station. This low turbine entry temperature (compared with an internal-combustion gas turbine) is why the Rankine cycle is often used as a bottoming cycle in combined cycle gas turbine power stations. The working fluid in a Rankine cycle follows a closed loop and is re-used continually. While many working fluids can and have been used in the Rankine cycle, water is usually the fluid of choice because it is abundant, inexpensive, nontoxic, generally non-reactive, and possesses favorable thermodynamic properties. Organic Rankine cycles have been developed to enable recovery of energy from lower temperature sources, such as industrial waste heat, geothermal heat, solar ponds, and so forth. The Organic Rankine cycle (ORC) is named for its use of an organic, high molecular mass fluid having a liquid-vapor phase change, or boiling point that occurs at a lower temperature than the water-steam phase change. Using the ORC, low-temperature heat can be converted to useful work, which, for example, can be harnessed to generate electricity. A prototype ORC power system was first developed and exhibited in 1961 by Israeli solar engineers Harry Zvi Tabor and Lucien Bronicki.
The organic Rankine cycle technology has many possible applications. Among them, the most widespread and promising fields are the following: waste heat recovery is the most important development field for the ORC. It can be applied to heat and power plants, or to industrial and farming processes such as organic products fermentation, hot exhausts from ovens or furnaces, flue gas condensation, exhaust gases from vehicles, intercooling of a compressor, and the condenser of a power cycle.
Biomass is available all over the world and can be used for the production of electricity on small to medium size scaled power plants. The problem of high specific investment costs for machinery such as steam boilers are overcome due to the low working pressures in ORC power plants. The ORC process also helps to overcome the relatively small amount of input fuel available in many regions because an efficient ORC power plant is possible for smaller sized plants.
Geothermic heat sources vary in temperature from 50° C. to 350° C. The ORC is, therefore, uniquely suited for this kind of application. However, it is important to keep in mind that for low-temperature geothermal sources (typically less than 100° C.), the efficiency is very low and depends strongly on heat sink temperature, which is typically the ambient temperature.
The ORC can also be used in the solar parabolic trough technology in place of the usual steam Rankine cycle. The ORC allows a lower collector temperature, a better collecting efficiency (reduced ambient losses) and, hence, the possibility of reducing the size of the solar field.
The selection of an appropriate working fluid is of key importance in low-temperature Rankine Cycles. Because of the low temperature, heat transfer inefficiencies are highly prejudicial. These inefficiencies depend very strongly on the thermodynamic characteristics of the fluid and on the operating conditions. In order to recover energy from low-grade heat sources, the working fluid must have a lower boiling temperature than water. Refrigerants and hydrocarbons are the two commonly used components. Unlike water, organic fluids usually suffer chemical deterioration and decomposition at high temperatures. The maximum hot source temperature is thus limited by the chemical stability of the working fluid. In addition, the freezing point should be lower than the lowest temperature in the cycle. A fluid with a high latent heat and density will absorb more energy from the source in the evaporator and thus reduce the required flow rate, the size of the facility, and energy consumption of the pump. Other important characteristics for an organic working fluid are that it has low ozone depletion and low global warming potential, that it be non-corrosive, non-flammable, non-toxic, in addition to being readily available at a reasonable cost.
On May 29, 2008, ElectraTherm, Inc. of Carson City, Nev. announced the successful installation of its first commercial waste heat generator at Southern Methodist University in Dallas, Tex. The generator, dubbed the “Green Machine,” makes electricity from residual industrial heat that has, heretofore, gone to waste. The U.S. Department of Energy reports that the available seven quadrillion Btu of waste heat sources exceeds the current production of all other U.S. renewable power sources combined. This includes hydroelectric, wood, biofuels, geothermal, wind, and solar photovoltaic. With a scalable output of 50 kW-500 kW and a subsidy-free payback period of less than three years, ElectraTherm's creation has the potential to significantly expand the production of electricity at very low cost at every fossil fuel burning power plant without burning additional oil, gas or coal, and without further pollution or damage to the environment. From liquids having temperatures as low as 93 degrees C., the process extracts heat to run a twin-screw expander, which is coupled to a generator. The company's twin-screw expander, which costs about one-tenth the price of a turbine, operates free of expensive gear boxes and electronics, runs at one-tenth the speed of turbines, operates with far less friction than does a turbine, and utilizes process lubrication without the need for a traditional oil pump, oil tank, oil lines and oil filter, enables the Green Machine to produce electricity at a cost of $0.03 to $0.04 per kW/hr during the payback period and for less than $0.01 per kW/hr thereafter. Although U.S. patent application Ser. No. 11/407,555, titled Waste Heat Recovery Generator, was filed by inventor Richard K. Langson on Apr. 19, 2006, covering the generator process and apparatus, with a priority date based on the filing of Provisional Patent Application No. 60/673,543, the application was finally rejected for obviousness in December of 2007. Langson also subsequently filed a related application titled Power Compounder, which covers certain aspects of the invention, and which issued as U.S. Pat. No. 7,637,108.